The invention relates to a hybrid drive for a motor vehicle with a combustion engine and a vehicle transmission with variable gear ratio and a first and second electrical machine which can be operated both as a motor and a generator, each of which comprises a stator and a rotor, with the second electrical machine being in a permanent nonpositive connection with an input of the vehicle transmission, with a shiftable clutch each being arranged between the electrical machines and the drive shaft of the combustion engine, and with the electrical machines to be connected with each other and/or an electrical energy source via an electronic control circuit.
Electric motors as drive motors for motor vehicles, which are supplied with electric energy by a battery or an on-board fuel cell permit emission-free and nearly silent driving, while being of a compact construction. The driving performance of purely electrically driven vehicles, however, is heavily restricted, due to the limited storage capacities of currently employed batteries, which is why electrovehicles have been employed only in special areas.
Apart from purely electric drive systems, partially electric drives are known which are also referred to as “hybrid drives”. These partially electrical drive systems essentially have a combustion engine as drive unit, by means of which a high performance and large cruising range of the motor vehicles are achieved. As a supplement, at least one electrical machine is arranged between the combustion engine and the transmission, which is arranged in series with or parallel to the drive train (between combustion engine and transmission) and offers the advantages such as for example brake energy recovery and emission-free driving.
In so-called serial hybrid drives, travelling is effected purely electrically with limited power in one driving range, i. e. the combustion engine and one of the two electrical machines are at standstill. The entire required energy is supplied from a battery. In a second driving range, travelling is effected by means of the combustion engine which drives the second electrical machine operating as a generator which in turn supplies the energy for the driving motor in the electric mode. Such serial hybrid drives operate without clutches in a permanent non-positive connection.
In so-called parallel hybrid drives, starting is effected purely electrically by means of an electric motor which is connected with a transmission input shaft. Meanwhile, the combustion engine is at standstill, which is disengaged from the transmission input shaft by a non-positive clutch. At a higher power requirement, e. g. exceeding a certain higher travelling speed, the combustion engine is started by the closing of a clutch, with the combustion engine then serving as a prime mover. The electric motor can then be used as an additional power source or a generator for charging the vehicle battery.